The subject technology generally relates to enzymes and methods for biosynthesis of styrene.
Styrene (vinyl benzene) is an organic compound with a chemical formula of C8H8. This cyclic hydrocarbon is a colorless, oily liquid that evaporates easily and has a sweet rubber-like smell. At higher concentrations, styrene confers a less pleasant odor. Styrene is named after the styrax trees (Styrax platanifolius) from which sap (a type of benzoin resin) can be extracted. Low levels of styrene occur naturally in several plant species. A variety of foods such as fruits, vegetables, nuts, beverages, and meats also contain styrene.
Industrially, styrene is the precursor to polystyrene and several copolymers. The presence of the vinyl group allows styrene to polymerize. Approximately 15 billion pounds are produced annually. The production of styrene in the United States increased dramatically during the 1940s, when it was popularized as a feedstock for synthetic rubber. Today, commercially significant products include polystyrene, acrylonitrile butadiene styrene (ABS), styrene-butadiene (SBR) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylene-styrene), styrene-divinylbenzene (S-DVB), styrene-acrylonitrile resin (SAN) and unsaturated polyesters. These materials are used in rubber, plastic, insulation, fiberglass, pipes, automobile and boat parts, food containers, and carpet backing.
Styrene is produced in industrial quantities mostly from ethylbenzene, which is in turn prepared on a large scale by alkylation of benzene with ethylene. It is one of the most important petrochemical products. There are several methods to produce styrene. Dehydrogenation of ethylbenzene is the most common way of production. Ethylbenzene is mixed in the gas phase with 10-15 times its volume in high-temperature steam, and passed over a solid catalyst bed. Most ethylbenzene dehydrogenation catalysts are based on iron(III) oxide, promoted by several percent potassium oxide or potassium carbonate. Steam serves several roles in this reaction. It is the source of heat for powering the endothermic reaction, and it removes coke that tends to form on the iron oxide catalyst through the water gas shift reaction. The potassium promoter enhances this decoking reaction. The steam also dilutes the reactant and products, shifting the position of chemical equilibrium towards products. A typical styrene plant consists of two or three reactors in series, which operate under vacuum to enhance the conversion and selectivity. Typical per-pass conversions are ca. 65% for two reactors and 70-75% for three reactors. Selectivity to styrene is 93-97%. The main byproducts are benzene and toluene. Because styrene and ethylbenzene have similar boiling points (145 and 136° C., respectively), their separation requires tall distillation towers and high return/reflux ratios. At its distillation temperatures, styrene tends to polymerize. To minimize this problem, early styrene plants added elemental sulfur to inhibit the polymerization. During the 1970s, new free radical inhibitors consisting of nitrated phenol-based retarders were developed. More recently, a number of additives have been developed that exhibit superior inhibition against polymerization.
Since styrene is an essential petrochemical used in many chemical products, alternative production methods, especially ones that do not require fossil fuels as feed stock, are urgently needed. Hence, despite the availability of methods for producing styrene, there is a continuing need for new methods for producing styrene monomers that are efficient and less expensive.